Process for coupling styrenic block copolymers

ABSTRACT

Disclosed is a process for coupling styrenic block copolymers comprising admixing vinyl cyclohexene dioxide with a living block copolymer comprising at least one block of polymerized vinyl aromatic hydrocarbon monomers and at least one block of polymerized conjugated diene monomers, at a molar ratio of from 0.40 to 0.60 and carrying out a coupling reaction at a temperature of at least 65° C. to produce a coupled styrenic block copolymer. Also disclosed is a polymer composition comprising at most 30% by weight of a styrenic block copolymer comprising at least one block A of polymerized vinyl aromatic monomer(s) and at least one block B of polymerized conjugated diene(s) and at least 70% by weight of the styrenic block copolymer coupled with vinyl cyclohexene dioxide.

This application is a 371 of PCT/US01/12074, filed Apr. 12, 2001 whichclaims the benefit of provisional application 60/197,966, filed Apr. 17,2000.

FIELD OF THE INVENTION

This invention relates to a method for producing styrenic blockcopolymers wherein living block copolymer chains are linked together byutilizing a linking compound which has at least two functional groups.More particularly, the present invention relates to a method forachieving high efficiency coupling of styrenic block copolymers by using4-vinyl-1-cyclohexene diepoxide as the linking compound to couple twoliving block copolymer chains together.

BACKGROUND OF THE INVENTION

The production of styrene-diene-styrene triblock copolymers by couplingof the styrene-diene diblock is well known and has a number ofadvantages over the production of corresponding polymers by fullysequential polymerization. Higher overall rates are achieved and moresymmetrical styrene blocks are formed since the coupling process avoidsthe crossover problem encountered when initiating a third styrene blockin the fully sequential polymerization method. Styrene block symmetry isimportant to a number of physical properties including tensile strength.

A large number of organic compounds have been reported as useful ascoupling agents for such block copolymers. The most commonly usedcoupling agent for the production of linear polymers is dibromoethane.Coupling efficiencies in the 80 to 85 percent range can easily beachieved with dibromoethane and the products have excellent meltviscosity stability. However, these products discolor after aging atelevated temperature due to the presence of lithium bromide. Othercoupling agents, including methyl formate and a variety of silanes, areknown to produce color stable linear polymers with reasonably highcoupling efficiency but most of the products exhibit significantly lowermelt viscosity stability.

U.S. Pat. No. 5,461,095 describes coupled polymers which are said tohave a high coupling efficiency, good melt stability, and also goodcolor stability. These polymers are said to have been produced by usingaromatic epoxy compounds such as high purity, high diepoxy content,diglycidyl ether of bisphenol A (DGEBA) epoxy resins as the couplingagent. EPON® 825 resin from Shell Chemical Company is one example ofthis class of resins. Unfortunately, these aromatic epoxies have verylow solubility in the aliphatic hydrocarbon solvents generally used foranionic polymerization of styrene-diene block copolymers, such ascyclohexane. One consequence of this is that efficient coupling is hardto achieve without vigorous mixing. These aromatic epoxy resins alsotend to be quite viscous, making precise metering, which is arequirement for high coupling efficiency, more difficult and, since thesolubility is low, dilution with a process solvent is not an option.

WO 99/01490 describes the use of glycidyl ethers of aliphaticpolyalcohols as coupling agents. While these materials have a lowerviscosity and good solubility in hydrocarbon solvents, commerciallyavailable grades are contaminated with high levels of monoepoxy andhydroxylic material. Purification by an expensive vacuum distillation isrequired before high coupling efficiency can be achieved.

4-vinyl-1-cyclohexene diepoxide (VCHD) is a low viscosity, hydrocarbonsoluble diepoxy compound that is commercially available in very highpurity (high diepoxy content). However, it has generally been consideredtoo unreactive to be of use in the production of linear comparative testV6,4-vinyl-1-cyclohexene diepoxide was used to couple a livingpolybutadiene polymer at 50° C. The coupling yield was only 50.7%. Inmost applications, it is desirable for the coupling efficiency to be atleast 70%, preferably close to 80%.

It can be seen that there is a need for a coupling agent forstyrene-diene block copolymers which will give a high couplingefficiency and also produce products which are highly melt stable andhave good color stability, and also exhibits relatively good solubilityin aliphatic hydrocarbon solvents. The present invention provides such acoupling agent and a process for achieving high coupling efficiency.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a process for coupling styrenicblock copolymers comprising admixing vinyl cyclohexene dioxide with aliving block copolymer comprising at least one block of polymerizedvinyl aromatic hydrocarbon monomers and at least one block ofpolymerized conjugated diene monomers, at a molar ratio of from 0.40 to0.60 and carrying out a coupling reaction at a temperature of at least65° C. to produce a coupled styrenic block copolymer.

In another aspect, the present invention is a polymer compositioncomprising at most 30% by weight of a styrenic block copolymercomprising at least one block A of polymerized vinyl aromatic monomer(s)and at least one block B of polymerized conjugated diene(s) and at least70% by weight of the styrenic block copolymer coupled with vinylcyclohexene dioxide.

This invention is a process for coupling styrenic block copolymersutilizing 4-vinyl-1-cyclohexene diepoxide (VCHD) as the coupling agent.First, styrene or another vinyl aromatic hydrocarbon is anionicallypolymerized to produce a living styrene polymer block of desiredmolecular weight. Next, a diene, such as butadiene or isoprene, isanionically polymerized onto the living end of the styrene polymerblock. This polymerization is carried out such that the diene block hasa molecular weight of one-half of the desired molecular weight of thefinal polymer. Next, 4-vinyl-1-cyclohexene diepoxide is added to thereaction mixture at a mole ratio of from 0.40 to 0.6, preferably 0.5 to0.55, moles of 4-vinyl-1-cyclohexene diepoxide per mole of polymer, andthe reaction is carried out at a temperature of at least 65° C.,preferably 75° C. to 95° C., for at least 15 minutes, preferably atleast 30 minutes.

It would be desirable in the art of preparing coupled styrene-dieneblock copolymers to prepare such copolymers using a process which has ahigh coupling efficiency. It would also be desirable in the art ofpreparing such copolymers if the copolymers could be used as or toprepare pressure sensitive adhesives and hot melt adhesives.

DETAILED DESCRIPTION OF THE INVENTION

As is well known, polymers containing both aromatic and ethylenicunsaturation can be prepared by copolymerizing one or more diolefins,particularly a diolefin, such as butadiene or isoprene, with one or morealkenyl aromatic hydrocarbon monomers, such as styrene. The copolymersmay, of course, be random, tapered, block or a combination of these, inthis case block. The products of this invention are preferably mixturesof at least 70% A-B-A triblocks, the remainder being A-B diblock,prepared by coupling A-B diblock copolymers, in which the terminalmonomer unit of the B block is derived from isoprene or butadiene. Thetwo resulting polymers may also be blended with other diblock ortriblock polymers or with star or radial polymers, i.e. polymers of thegeneral structure (A-B)_(n)-X, where n is greater than 2. Mixed polymerstructures can also be made by coupling with a mixture of4-vinyl-1-cyclohexene diepoxide and other coupling agents.

Polymers of conjugated diolefins and copolymers of one or moreconjugated diolefins and one or more alkenyl aromatic hydrocarbonmonomers are frequently prepared in solution using anionicpolymerization techniques. In general, when solution anionic techniquesare used, these block copolymers are prepared by contacting the monomersto be polymerized simultaneously or sequentially with an organoalkalimetal compound in a suitable solvent at a temperature within the rangefrom about −150° C. to about 300° C., preferably at a temperature withinthe range from about 0° C. to 100° C. Particularly effective anionicpolymerization initiators are organolithium compounds having the generalformula:

RLi_(n)

Wherein:

R is an aliphatic, cycloaliphatic, aromatic or alkyl-substitutedaromatic hydrocarbon radical having from 1 to about 20 carbon atoms; andn is an integer of 1 to 4.

The molar ratio of the initiator to the monomer determines the blocksize, i.e. the higher the ratio of initiator to monomer the smaller themolecular weight of the block. For products in the molecular weightrange of interest, this generally leads to an initiator concentration inthe range of about 0.25 to about 50 millimoles per 100 grams of monomer.The monomer concentration (solids) is generally dictated by thelimitations placed on the viscosity of the resulting polymer solutionand heat removal during the polymerization. Polymerization solids aregenerally in the range of 5 percent by weight (% wt) and 50% wt,preferably, 20% wt and 40% wt.

In general, any of the solvents known in the prior art to be useful inthe preparation of such polymers may be used. Suitable solvents, then,include straight- and branched-chain hydrocarbons such as pentane,hexane, heptane, octane and the like, as well as, alkyl-substitutedderivatives thereof; cycloaliphatic hydrocarbons such as cyclopentane,cyclohexane, cycloheptane and the like, as well as, alkyl-substitutedderivatives thereof; aromatic and alkyl-substituted aromatichydrocarbons such as benzene, naphthalene, toluene, zylene and the like;hydrogenated aromatic hydrocarbons such as tetralin, decalin and thelike. Further, polar cosolvents may be used. Suitable cosolvents includelinear and cyclic ethers such as methyl, ether, methyl ethyl ether,tetrahydrofuran and the like, and mixture of such ethers.

As described in U.S. Pat. No. 4,096,203 the disclosure of which isherein incorporated by reference, usually the styrene is contacted withthe initiator. Next, the living polymer in solution is contacted withisoprene or another diene. The resulting living polymer has a simplifiedstructure A-B-Li. It is at this point that the living polymer is coupledwith a coupling agent.

In the prior art, such as that exemplified by U.S. Pat. Nos. 3,595,941and 3,468,972, the disclosures of which are herein incorporated byreference, the effort was always made to select the particular couplingagent or reaction conditions that resulted in the highest couplingefficiency. High coupling efficiencies are desired herein as well inorder to produce more A-B-A polymer. Coupling efficiency is defined asthe number of molecules of coupled polymer divided by the number ofmolecules of coupled polymer plus the number of molecules of uncoupledpolymer. In the present invention, the use of 4-vinyl-1-cyclohexenediepoxide as the coupling agent allows a coupling efficiency of at least70 percent to be achieved.$\frac{\# \quad {of}\quad {molecules}\quad {of}\quad ({SI})_{2}}{\# \quad {of}\quad {molecules}\quad {of}\quad ({SI})_{2}\quad {plus}\quad {SI}}$

Coupling efficiency is usually determined by gel permeationchromatography (GPC).

It is not sufficient to achieve the desired result of the presentinvention to merely use 4-vinyl-1-cyclohexene diepoxide as the couplingagent. The coupling process must be carried out under specifiedconditions. First, the temperature of the coupling reaction must bemaintained at at least 65° C. and preferably at 75 to 95° C. If thetemperature is lower than 65° C. then the coupling efficiency which canbe achieved is much lower, i.e., such as the 50 percent couplingefficiency which was achieved in WO 99/01490. The temperature probablyshould not be higher than 95° C. because thermal termination of some ofthe living polymer blocks may occur prior to coupling.

The coupling reaction is preferably carried out for at least 15 minutesand preferably at least 30 minutes. If the coupling reaction time isless than 15 minutes, then the overall coupling efficiency goal will notbe achieved. It is probably not necessary to carry out the couplingreaction for more than 60 minutes because the small additional couplingthat takes place is not justified by the cost of the increased time ofreaction.

Finally, it is important to achieve the correct stoichiometry. Ideally,it would be expected that the maximum coupling efficiency would beachieved for a difunctional coupling agent when the molar ratio of thecoupling agent to the polymeric lithium anions is exactly 0.5. Inpractice, there may be side reactions that not only limit the maximumachievable coupling, but also influence this ratio. In the case of4-vinyl-1-cyclohexene diepoxide, it is preferable to err on the side ofexcess diepoxide. Coupling efficiencies of at least 70% can be achievedif this ratio is between 0.40 and 0.6; high coupling efficiencies aremost reliably achieved when this ratio is between 0.5 and 0.55.

Following the coupling reaction or when the desired coupling efficiencyhas been obtained, the product is neutralized such as by the addition ofterminators, e.g., hydrogen, water, alcohol or other reagents, for thepurpose of terminating residual active anions. The product is thenrecovered such as by coagulation utilizing hot water or steam or both.

The coupled polymer is then finished and shipped off to the end user.The polymer is then combined with other components to form whatever enduse composition is desired.

EXAMPLES

The polymers made were homopolymers of butadiene or isoprene. Thefollowing polymerizations were carried out in a two-liter Buchi glassautoclave reactor according to the following general procedure. Thedesired cyclohexane solvent charge was added, followed by 100 grams ofmonomer and any modifiers or cosolvents desired to be present duringpolymerization. The solvent charge was adjusted such that the reactorcontents at the end of polymerization would not exceed 1000 grams. ALauda bath was used to apply heating and cooling to the reactor jacket.The reactor contents were then heated to the initiation temperature,usually 40° C. Once the initiation temperature was reached, the desiredquantity of s-butyl lithium was added and 44 grams of dry cyclohexanewere flushed to ensure complete transfer. If the polymerization wasbeing run at 20 percent solids, a second 100 grams charge of monomer wasadded when the exothermic temperature rise peaked. When the exothermictemperature rise from the final monomer charge had peaked, the bathtemperature was set to the desired temperature for coupling. The4-vinyl-1-cyclohexene diepoxide coupling agent was added when thereactor had reached the desired temperature and the polymerization hadbeen allowed to proceed for at least eight half-lives.4-Vinyl-1-cyclohexene diepoxide was obtained from Union Carbide(ERL-4206) and used as received or dried over 4A molecular sieves. Thecoupling agent was added as a 10 percent weight solution in drycyclohexane and 44 grams of dry cyclohexane was flushed through toensure complete transfer. At the end of the coupling reaction, methanol(1 mole per mole of s-butyl lithium) was added and the reactor contentswere transferred to a glass bottle for storage.

Reactions in which the coupling temperature was greater than 85° C. werecarried out in a one-liter stainless steel ZipperClave (AutoclaveEngineers) autoclave reactor equipped with electronic heating. The totalweight charged at the end of the polymerization was adjusted to 625grams. The initiation temperature was increased to 65° C. withoutincurring too great of an exothermic temperature increase due to theimproved heat transfer in the reactor. Otherwise, the polymerizationswere carried out as above.

Some experiments were carried out in a larger reaction facility. Thepolymerizations were carried out according to the same general procedureas above. However the target molecule in these experiments was a highmolecular weight Styrene-Isoprene-Styrene triblock copolymer. First,polystyryllithium of a number average molecular weight of about 10,800amu was prepared by contacting styrene and s-butyllithium incyclohexane. Isoprene and cyclohexane were charged into a secondreactor, and then a portion of the polystyryllithium solution wastransferred to the second reactor. The amount of isoprene and polymerwas chosen so that the resulting diblock would have an overall numberaverage molecular weight of about 67,000 amu and a polystyrene contentof about 15% wt, and the solids at the end of the reaction would beabout 20% wt. The coupling agent was added neat, after the isoprenepolymerization was complete.

Example 1

Living polyisoprene homopolymer molecules were reacted with4-vinyl-1-cyclohexene diepoxide at a ratio of about 0.5 moles ofcoupling agent per mole of living polymer at temperatures between 50° C.and 95° C. in cyclohexane. In all of the examples, samples were taken15, 30, and 60 minutes after the coupling agent was added. In somecases, the reactions were allowed to proceed for a total of 4 hours, andsamples were taken every hour. The results are summarized in Table 1below. The Coupling Agent: Polymer-Li ratios in the table belowrepresent the average of values obtained using the charge data and (1)the number of average molecular weight (M_(n)) as determined by GPC, (2)the number average molecular weight (M_(n)) as determined by ¹H NuclearMagnetic Resonance (NMR), and (3) the diepoxy: s-butyl ratio determineddirectly by ¹H NMR.

TABLE 1 Molecular Temp Reaction Observed Run # Weight (° C.) Time (min)CA:PLi CE 23838-168 10,000 50 60 0.54 49% 24372-15 10,000 75 60 0.58 77%24372-19 10,000 75 60 0.59 76% 24372-23 10,000 75 60 0.58 78% 24372-2510,000 75 60 0.59 74% 24372-37 10,000 75 60 0.42 77% 24372-39 10,000 7560 0.49 78% 24372-41 10,000 75 60 0.48 76% 24372-35  1,500 75 60 0.5085% 24372-49  2,500 75 30 0.42 74% 24372-51  2,500 75 15 0.49 67%24372-61  2,500 75 240 0.46 78% 24372-81  2,500 85 240 0.45 81%24372-103  2,500 95 240 0.52 81%

It can clearly be seen that coupling efficiencies within the desirerange can be achieved by carrying out the reaction at temperatureshigher than 50° C. for at least 15 minutes with coupling agent:polymeric lithium ratios in the range of 0.45-0.6. Close inspection ofthe data reveals that coupling efficiencies on the high end of the range(77%-78%) were obtained at ratios as high as 0.58.

The change in coupling efficiency as a function of time at 75° C., 85°C., and 95° C. is summarized in Table 2 below. At all temperaturesexamined, the majority of coupling occurs in the first 15 minutes. At85° C. and 95° C., the reaction is essentially complete after 30minutes.

TABLE 2 Temperature Reaction Observed Run # (° C.) CA:PLi Time (min.) CE24372-61 75 0.46 1 54% 5 66% 20 74% 30 75% 60 77% 120 78% 180 78% 24078% 24372-81 85 0.45 1 63% 5 75% 15 79% 30 80% 60 81% 120 81% 180 81%240 81% 24372-103 95 0.52 1 64% 5 80% 15 81% 30 82% 60 82% 120 82% 24081%

The runs described in Table 3 were for styrene-isoprene-styrene blockcopolymers and were carried out in the larger facility. Due to the highmolecular weight of the products, we were unable to directly determinethe Coupling Agent: Polymer-Li ratio by ¹H NMR. The values reported herewere calculated from the charges and the relevant molecular weights asdetermined by GPC.

TABLE 3 Run # CA:PLi Temp (° C.) Observed CE 7328D 0.50 75 80% 7333D0.49 75 76% 7333D 0.49 75 79% 7340D 0.50 65 78% 7344D 0.48 85 79% 7368D0.42 75 70% 7555M 0.49 75 74%

All of these experiments were carried out at temperatures within thetemperature range of the present invention. Also, the coupling agent toliving chain end molar ratios were within the range of the presentinvention. In each case, a coupling efficiency of at least 70% wasachieved. Clearly, products typical of commercial styrenic blockcopolymers can be prepared using this technology.

Example 2

Several more polymerizations were carried out using polar cosolventwhich are often used in the polymerization of styrene-diene blockcopolymers. In these example, o-dimethoxy benzene (ODMB), diethoxypropane (DEP), and diethyl ether (DEE) were used as modifiers in thepolymerizations. Isoprene or butadiene were polymerized to a numberaverage molecular weight of about 10,000 in cyclohexane. Next, the vinylcyclohexene diepoxide was added. The reactions were carried out at 65°C. and 75° C. for 60 minutes. The results are shown in Table 4 below.The coupling data for the polybutadiene in cyclohexane was consistentwith that which was observed for polyisoprene in the previous examples.

TABLE 4 Terminal Temp Coupling Run # Monomer Modifier (° C.) CA/PLiEfficiency 24372-33 Isoprene 400 PPM ODMB 75 0.50 72% 24372-47 Isoprene300 PPM DEP 75 0.51 75% 24372-31 Butadiene None 75 0.50 70% 24372-57Butadiene 6% DEE 65 0.48 79%

Example 3

The product of Run @7555 was finished in a pilot-scale hot watercoagulator. The resulting crumb was further dried under vacuum at 40° C.for about one week. Samples were also produced by either drying the asreceived cement from Run @7555 at about 40° C. under vacuum overnight,or adding 0.1% wt deionized water to the cement prior to drying. In thehot water coagulated sample, roughly ⅓ of the lithium is expected to beextracted from the polymer. The samples recovered by drying the cementwill contain all of the lithium. In the case where water is added, themajority of the lithium is expected to be converted to hydroxide orcarbonate prior to exposure of the sample to heat.

The melt viscosity stability and high temperature color stability wasassessed as follows. Samples of the dry polymer were formulation intoadhesives of the following composition:

25% wt Polymer

60% wt Escorex® 5300 tackifying resin

15% wt Tufflo® oil

2 phr Irganox® 1010 antioxidant

wherein phr means parts per hundred parts of rubber.

The adhesives were heated in an oven and exposed to air at 350° F. (177°C.) for the time period noted in the tables below. The viscosity wasmeasured at 177° C. using a Brookfield rheometer and the color wasassessed using the Gardner method. The results are summarized in Tables5 and 6. Comparative Examples A and B represent typical data for thesame type of polymer coupled with a DGEBA epoxy (EPON® 825 resin) andethylene dibromide (EDB), respectively. The melt viscosity stability isa measure of how well the polymer's structure remains intact at hightemperatures. The melt stability of the VCHD coupled product is clearlycomparable to that of the EPON® 825 resin and EDG coupled products.Further, the high temperature color stability is comparable to that ofthe EPON® 825 resin coupled product, and clearly superior to that of theEDB coupled material.

TABLE 5 Viscosity (Centipoise/Pa · s) & Percent Change After: Initial 8Hours 24 Hours 48 Hours Sample Viscosity Viscosity Decrease ViscosityDecrease Viscosity Decrease 7555 (Coagulated) 4790/4.79 4032/4.03 16%2438/2.44 49% 1180/1.18 75% 7555 (Lab Dried) 5520/5.52 4952/4.95 10%2925/2.92 47% 950.0/0.95 83% 7555 (0.1% H₂O Lab Dried) 5210/5.214315/4.32 17% 2670/2.67 49% 992/0.99 81% Comparative Example A 7560/7.566470/6.47 14% 2490/2.49 67% 1180/1.12 84% Comparative Example B6350/6.35 5370/5.37 15% 2920/2.92 54% 680/0.68 89%

TABLE 6 Gardner Color After: Sample Initial 8 Hours 24 Hours 48 Hours 96Hours 7555 (Coagulated) 1 3 5 8 10 7555 (Lab Dried) 1 2 4 7 10 7555(0.1% 1 1 3 6 9 H₂O Lab Dried) Comparative 1 2 3 5 7 Example AComparative 1 4 9 14 15 Example B

We claim:
 1. A process for coupling styrenic block copolymers comprisingadmixing vinyl cyclohexene dioxide with a living block copolymercomprising at least one block of polymerized vinyl aromatic hydrocarbonmonomers and at least one block of polymerized conjugated dienemonomers, at a molar ratio of from 0.40 to 0.60 and carrying out acoupling reaction at a temperature of at least 65° C. to produce acoupled styrenic block copolymer.
 2. The process of claim 1 wherein thecoupling reaction is carried out for at least 15 minutes.
 3. The processof claim 1 wherein the solvent is a mixture of hydrocarbon solvent and apolar cosolvent.
 4. The process of claim 1 wherein the molar ratio is0.5 to 0.55.
 5. The process of claim 1 wherein the temperature is from75 to 95° C.
 6. The process of claim 1 wherein the coupling reaction iscarried out for at least 30 minutes.
 7. The process of claim 1 whereinan A-B-A triblock copolymer is prepared by coupling an A-B diblockcopolymer.
 8. The process of claim 7 wherein the terminal monomer unitof the B block is derived from isoprene or butadiene.
 9. A polymercomposition comprising at most 30% by weight of a styrenic blockcopolymer comprising at least one block A of polymerized vinyl aromaticmonomer(s) and at least one block B of polymerized conjugated diene(s)and at least 70% by weight of the styrenic block copolymer coupled withvinyl cyclohexene dioxide.